One of known optical encoders comprises three grating scale plates. In this optical encoder, the three scale plates are disposed in order along a light traveling direction, with the main surfaces of the scale plates in parallel with one another so that the gratings on the scale plates can be arrayed in the same direction. A low spatially coherent light source is disposed in the front of the first scale plate, and a light-receiving element which converts a light quantity into an electric signal is disposed in the rear of the third scale plate.
The operation of this optical encoder is disclosed in Non-Patent Literature 1. According to this Literature, when the light source illuminates the first scale plate, secondary light sources having low spatial coherences, arrayed at a certain cycle and having a multislit-like light quantity distribution, are formed. The second scale plate acts as a spatial frequency filter having a certain optical transmission function (or OTF) and extracts only a specific spatial frequency component from the light quantity distribution of the secondary light sources, so that such a specific spatial frequency component can be formed as an image on the third scale plate. The light beams having passed through the light transmitting portions of the grating provided on the third scale plate are converted into an electric signal by the light-receiving element. When the first scale plate or the second scale plate is relatively moved along the grating-arraying directions, an output signal having a correlation to this relative position is obtained.
For example, when the light quantities of the secondary light sources formed by the first scale plate are distributed like a sine wave with a certain cycle P, and when the optical transmission function of the grating provided on the second scale plate contains a spatial frequency corresponding to the cycle P, an image having a sine wave-like light quantity distribution with the cycle P is formed on the third scale plate.
In this case, the optical transmission function of the second scale plate varies depending on the grating cycle of the second scale plate, the shapes of the slits of the grating, the shape of the grating itself (in case of a phase grating), the interval between the second scale plate and the first scale plate and the interval between the second scale plate and the third scale plate. In general, an optical encoder is designed by selecting conditions which sharpen the contrast of a light quantity distribution formed as an image or a third scale plate.
An optical encoder disclosed in Patent Literature 1 comprises three scale plates each of which has thereon an amplitude grating having rectangular slits, wherein the interval between the first scale plate and the second scale plate is equal to the interval between the second scale plate and the third scale plate, and wherein the grating cycles of the first and third scale plates are designed to be 2P which is twice larger than the grating cycle P of the second scale plate.
An optical encoder disclosed in Patent Literature 2 employs a first scale plate having thereon a phase grating which generates a shading pattern by way of a light interference phenomenon.
In each of the optical encoders disclosed in Patent Literature 1 and Patent Literature 2, the third scale plate and the light-receiving element disposed on the rear of the third scale plate may be replaced with a light-receiving element array which has a light-receiving portions with the same dimensions as the dimensions of the slits of the grating on the third scale plate, so that this light-receiving element array concurrently can have the functions or the third scale plate and the light-receiving element.                Patent Literature 1: JP-A-63-153408 (1988)        Patent Literature 2: JP-A-01-2761 (1998)        Non-Patent Literature 1: K. Hane and C. P Grover, “Imaging with rectangular transmission gratings”, J. Opt. Soc. Am. A4, No. 4, pp 706-711, 1987        